I. Field of the Invention
This invention relates generally to digital frequency divider apparatus and more specifically to a frequency divider which will convert the output from a crystal controlled oscillator utilizing a commonly available crystal as its operative element to produce an output pulse train which is compatible with the decimal number system.
II. Description of the Prior Art
Many types of frequency dividers are known in the digital computing and signal processing arts for converting the frequency of the output from a pulse source to a desired lower frequency. The simplest form of divider for pulse type signals is a bistable flip-flop stage which, when coupled to the pulse source, effectively divides the frequency of this source by two. By cascading plural flip-flop stages, one obtains frequency division by a factor of 2.sup.N, where N is the number of bistable stages utilized.
In the digital watch industry, which has experienced drastic growth since the advent of large scale integration of semiconductor devices, a common pulse source utilized is a stable oscillator having a crystal as its frequency determining element. The digital watch industry has generally standardized on a crystal having a natural frequency of 32.768 KHz. When the output from such an oscillator is applied to a 15 stage binary counter, the frequency of the incoming signal is effectively divided by 2.sup.15 or 32,768 to produce one pulse every second. Because crystals having a natural frequency of 32.768 KHz are so commonly used in large quantities in the digital watch field, economies of scale have resulted and such crystals may be obtained at relatively modest cost. Then too, because of their ready availability, their use in electronic devices other than digital chronometers would be advantageous from a manufacturing standpoint, provided a frequency divider can be devised to produce signals of a frequency compatible with these other devices.
For example, there is disclosed in the David J. Fischer application Ser. No. 724,019, filed Sept. 16, 1976 and entitled "PROGRAMMABLE DEMAND PACER" a cardiac pacemaker which makes extensive use of digital devices. Timing intervals are established in this pacer unit by driving one or more counters from a digital clock source. In this device, as well as in many others which may be envisioned, it is desirable to produce clock pulses at a frequency which is a predetermined power of 10, e.g., 10 KHz, 1 KHz, etc. If one is to utilize a crystal oscillator having the commonly available digital watch-type crystal used therein, it is desirable to have a frequency divider which will effectively divide the oscillator output by a factor which is other than a power of 2. Specifically, in order to have a clock signal of a frequency of 10 KHz, it would be necessary to frequency divide by a factor of 3.2768.
Many circuits have been devised for frequency dividing by various factors. For example, the Pugh U.S. Pat. No. 3,189,832 describes a digital circuit for frequency dividing by a factor (N+1/2) where N is a whole number. Similarly, the Andrea U.S. Pat. No. 3,571,728 describes a frequency divider for effecting division by a factor of N/2 where N is any integer. The Kokado U.S. Pat. No. 3,896,387 illustrates a circuit arrangement for frequency dividing by a factor of 2/N where N is an odd integer. The Patents to Fletcher et al. (U.S. Pat. No. 3,906,374), McGuffin (U.S. Pat. No. 3,943,379), Green (U.S. Pat. No. 3,982,199) and Chiapparoli (U.S. Pat. No. 4,041,403) teach digital circuits for frequency dividing by various other factors.
None of the above-described prior art patents, however, teaches or suggests the way in which a pulse train having a frequency which is an integral power of 2 may be frequency divided to produce a lower frequency pulse train which is an integral power of 10. Thus, the prior art has failed to solve the problem of how one might utilize a crystal controlled oscillator which incorporates the commonly available 32.768 KHz crystal ih an electronic device which is designed to employ a clock source operating in the base 10 system.